[TEST] Remove YAML Tests (elastic#131434)

Removes YAML tests for the `/_cluster/allocation/explain` API. The tests
 passed in alternate values for the APIs. An example is passing "true"
 for fields expecting a boolean value. While this is explicitly
 supported by the API, this is not the correct place to be testing this
 behaviour, and resulted in the API specification failing
 validation.

Relates elastic#127028

Author: joshua-adams-1